System formed by the combination of a solid state image intensifier and a compatible adapted x-ray film

ABSTRACT

An X-ray image intensifying device comprising A. A supporting film base, B. A conducting layer which is at least partially transparent to X-rays, C. An X-ray sensitive photoconductive layer, D. A light opaque layer E. An electroluminescent layer comprising an electroluminescent compound dispersed in a binder, F. An at least partially light-transparent electrically conducting layer, and G. A protective layer whose thickness is such that the resolving power loss is not greater than 20 percent lower than that of the same element without any protective layer, enables marked sensitivity increases in X-ray photographic systems.

United States Patent 1191 1111 3,870,892

Fass et al. Mar. 11, 1975 [5 1 SYSTEM FORMED BY THE COMBINATION3,210,551 10/1905 Vaughn et al. 250/484 OF A SOL") STATE [MACElNTENSlFlER 3,350,506 l()/l967 Chernow 250/2l3 R AND A COMPATIBLEADAPTED X RAY l Moulton 250/213 A FILM Inventors: Leonard Fass, AlbisolaSuperiore;

Ennio Fatuzzo, Ferrania, both of Italy Minnesota Mining andManufacturing Company, St. Paul, Minn.

Filed: Jan. 26, 1973 Appl. No.: 326,741

Assignee:

Foreign Application Priority Data Feb. 2, 1972 ltaly.... 48ll4/72References Cited UNITE? STATES PATENTS Primary Examiner-Harold A. DixonAttorney, Agent, or Firm-Alexander, Sell, Steldt & DeLal-lunt [57]ABSTRACT An X-ray image intensifying device comprising A. A supportingfilm base,

B. A conducting layer which is at least partially transparent to X-rays,

C. An X-ray sensitive photoconductive layer,

D. A light opaque layer,

B. An electroluminescent layer comprising an electroluminescent compounddispersed in a binder,

F. An at least partially light-transparent electrically conductinglayer, and

G. A protective layer whose thickness is such that the resolving powerloss is not greater than 20 percent lower than that of the same elementwithout any protective layer, enables marked sensitivity increases inX-ray photographic systems.

10 Claims, 2 Drawing Figures Snavcly- 250/484 The present inventionrelates to a new improved system for radiography.

Particularly it relates to a device which transforms informationcontained in an X-ray beam, whose intensity varies from point to pointin space, into a light beam, whose intensity variation corresponds tointensity variations of the incident X-rays, such a device beingcombined with a silver halide X-ray film whose spectral sensitivity issuitably selected to efficiently record the radiations emitted by theabove-mentioned device. Moreover, the above-mentioned device can consistof a so-called solid state image intensifier.

Since X-ray films are very insensitive to X-rays, to prevent excessiveradiation doses to the patient, socalled intensifying screens consistingof an X-ray fluorescent salt, such as calcium tungstate, dispersed in asuitable binder and coated on an X-ray transparent base, were until nowgenerally usedto obtain X-ray images in medical radiography. Suchscreens are positioned in close contact with an X-ray film in acontainer known as a cassette.

We have now found that the radiation doses to the patient can beremarkably decreased, that is a higher real sensitivity than thatachieved with the fluorescent intensifying screen X-ray film system canbe obtained by using a solid state flatX-ray image intensifying elementsuitable for radiography consisting of various layers arranged in thefollowing way:

a. a supporting film base b. an at least partially transparent to X-raysconducting layer c. an X-ray sensitive photoconductive layer d. a lightopaque layer e. an electroluminescent layer formed by anelectroluminescent compound dispersed in a binder f. an at leastpartially light-transparent conducting layer g. a protective layer whosethickness is such that the resolving power loss is not greater thatpercent lower than that of the same'element without any protectivelayer.

The above mentioned-sensitivity increase can be partially or whollyexploited to obtain a higher resolving power. This can be achieved byusing for instance X-ray film having much finer grains than standardX-ray films. This slightly decreases the high sensitivity of the system,but, on the other hand, allows a better resolving power to be obtained.

Such a sensitivity increase can be also exploited by using an X-ray filmhaving a lower silver halide quantity per unit surface area, with aconsequent reduction in manufacturing costs and processing times.

The higher sensitivity of the system can be finally exploited by usingX-ray films having emulsion coated on only one surface of the base, witha consequent reduction in manufacturing costs, simplification of theprocessing procedures and of the equipment necessary for the saidprocessing and the possibility of remarkably shortened processing times.

, As there is no conventional standard for the measureinent of the X-rayintensifying screen sensitivity, as a specimen screen or comparisonscreen we used a screen or, more precisely, a pair of medium sensitivityscreens of E. l. DuPont de Nemours and Co., Inc., of

Wilmington, Del., and placed in a conventional container or X-raycassette. Such an X-ray film together with the intensifying screens willbe hereinafter called the conventional system.

A preferred embodiment to obtain such an image intensifier consists ofpreparing a multi-layer structure formed by the following layers:

1. a supporting base, at least percent transparent to X-rays, formed forexample of aluminum foil or polyester of thickness not larger than about300 microns;

2. an electrically conductive layer which is at least 90 percenttransmissive of 20 150 kV X-rays. Such layer need not be applied in thecase when the supporting base is already conductive. Such a layer can becomposed of a metal or other conducting compound deposited by knowntechniques such as vacuum evaporation, sputtering, spraying, etc.

3. an X-ray sensitive photoconductive layer, the resistance of whichvaries according to the incident X-ray intensity and which resistancevaries by at least a factor of 10 when irradiated by X-rays produced bya tungstate target X-ray tube such as the Machlett OEG/50/T,manufactured by The Machlett Laboratories Incorporated, Springdale,Conn., such tube operated at 40 kV and lmA at a distance of 20 cm. Theradiation from the above tube is emitted through a 0.040 in. thickberyllium window with a 5 mm. focal spot. Suitable X-ray photoconductorsare CdS or CdSe or CdS Se doped with copper and in certain cases alsowith other elements such as Al, B, Cl etc., in the form of powders. Thepreparation of such photoconductors is described in ThePhotoconductivity of Solids by R. Bube, published by Wiley. The layer isprepared by coating the above mentioned photoconducting powdersdispersed in a low dielectric constant binder such as an acrylic orvinylic resin. Such photoconductive layer must be of a thickness (e.g.,microns) so as to give acceptable resolution for radiography.

4. A light opaque layer having an impedance lower than and preferably atleast a factor of 10 belowthat of the electroluminescent layer. In apreferred embodiment this layer must be of a highly lightdiffusingmaterial, such as particles of BaTiO TiO or other such compounddispersed in a high dielectric constant medium such as Cyanocel, achemically modified cellulose (highly cyanoethylated cellulose), made bythe American Cyanamid Company of Connecticut (U.S.A.). This layer mustbe of such a thickness that its impedance meets the requirementsspecified above with respect to the impedance of the electroluminescentlayer to be described in (5) below:

5. An electroluminescent layer having a dielectric constant of at least5 and having an impedance normally lower, but not less than 10 percentof that of the non-irradiated photo-conductive layer. This layer isformed by an electroluminescent compound such as commercially availableZnS doped with Cu and/or Ag dispersed in a binder. In a preferredembodiment the dielectric constant of the layer is at least 5, a resultwhich can be obtained for example by using a high dielectric constantmedium, such as Cyanocel, described in (4) above, as the binder.

6. an electrically conducting layer which is at least 50 percenttransmissive to light at wavelengths between 500 and 550 nm. This layeris preferably composed of a thin film of metal or other conductingcompound deposited by known vacuum techniques such as evaporation,sputtering, etc., as for example described in Vacuum Deposition of ThinFilms by L. Holland, published by Chapman and Hall Ltd. 1960.

7. A protective layer whose thickness is such that the resolving powerloss is not greater than percent lower than that of the same elementwithout any protective layer. Such layer can be composed of apolyurethane resin such as Desmophen 650 -Desmodur N (where Desmophen650 is 65 percent solution in ethylene glycol ofa branched polyesterwhich contains approximately 5 percent of hydroxyl groups, having anacidity value less than 4, a viscosity of approximately 800 i 150 c? anda fire point greater than 50C (DIN 51755) and Desmodur N is a 75 percentsolution in ethylene glycol acetate/xylene (1:1) of a polyfunctionalaliphatic isocyanate having an NCO content of 16-17 percent with a firepoint greater than 33C (DIN 53213), density at 20C of 1.06 gm/c.c. (DIN51757), viscosity at 20C of 250 :t 100 cP, of Farbenfabriken Bayer AG,Leverkusen, Germany). Another possible protective layer can be made froma vinyl chloro-acetate copolymer such as Vinylite VAGH (approximatelycontaining 91 percent vinyl chloride, 2.3 percent hydroxyl groups, 3percent vinyl acetate and having an intrinsic viscosity in cyclohexanoneof approximately 0.55 at 20C) of the Bakelite Division, Union Carbideand Carbon Corporation, New York, N.Y., USA, hardened with one part ofan isocyanate (Desmodur N) for every two parts of Vinylite VAGH. Yetanother protective layercan be made from a polyvinylidene chloridecopolymer such as that manufactured by the Dow Chemical Corporation ofDelaware (USA), under the name of Saran.

In another embodiment the protective layer described in (7) abovebecomes the supporting base and the other layers are sequentially coatedin reverse order to the previous embodiment.

It may be necessary to include a series of intermediate layers to obtainthe suitable conditions of electrical contact between the various layersmaking up the intensifier.

For the purposes of understanding this invention, the photoconductivelayer can be considered as being represented by a capacitance and aresistance in parallel, whereby the value of the former is not affectedby the X-rays, while the value of the latter is decreased under X-rayirradiation. The electro-luminescent layer on the other hand, can berepresented by a capacitance having a very high leakage resistance. Theimpedance of the unexposed photoconductor at a frequency equal to thefrequency of the applied alternating voltage must be higher than that ofthe electroluminescent layer. The impedance of the X-ray exposedphotoconductor (at X-ray exposures such as those described in (3) above)must be as small as possible and in any case not much larger than twicethe impedance of the electroluminescent layer.

The above implies that the capacitive part of the impedance of thephotoconductor must be as large as possible as compared to theresistance of the photoconductor and in any case not significantlysmaller than said resistance. This condition can be fulfilled by using alow dielectric constant binder for the photoconductive powder. Suchbinders can consist for example of acrylic or vinylic resins.

Another type of image intensifier useful for the scope of the presentinvention can be based on the principle described in US. Pat. No.3,215,847 with proper modifications to make it suitable for use inradiography together with a silver halide X-ray film, whose spectralsensitivity has been suitably selected to efficiently record theradiation generated by the above mentioned device.

A series of patents, such as US. Pats. Nos. 3,264,479; 3,300,645;3,215,847 and 3,394,261, relate to image intensifiers which transform asignal consisting of a suitably modulated X-ray beam into a light beam,whose modulation corresponds to the X-ray modulation. Such imageintensifiers however are not suitable for radiography but only forradio-scopy. In these intensifiers, generally, light is emitted througha very thick front protection panel (such as for instance through arelatively thick glass plate, 1 mm or greater). Therefore, such panelsare not suitable for contact recording with an X-ray film clue to theloss of resolution. Furthermore, it is not always possible to use themost suitable photoconductors in the prior art type image intensifiers,because often they have a too long a time constant.

According to the present invention, on the contrary, the electrode andthe protective layer are sufficiently thin to allow the recording of theimage formed on the electroluminescent compound, substantially withoutany loss of resolving power. Furthermore, in the present invention themost suitable photoconducting compounds can be used even if they have atime constant too long to be used in radioscopy.

FIG. 2 illustrates an X-ray intensifying device of the presentinvention. E represents the film base, F a conductive layer, G aphotoconductive layer, H a light opaque layer, 1 an electroluminescentlayer, J a conducting layer, and K represents the thin surfaceprotective layer whose thickness is such that the resolving power lossis not greater that 20 percent lower than that of the same elementwithout any protective layer.

As mentioned above, the higher sensitivity of the system, which can beobtained by using the intensifier of the present invention, can bepartially or wholly used to obtain a better resolving power andtherefore a clearer image, or can be used with an X-ray film having alower silver coating weight, thus allowing lower production costs andshorter processing times.

By using an X-ray film with a silver bromo-iodide emulsion having agrain size of 0.75 microns, a part of the remarkable sensitivityincrease of the system according to the present invention is sacrificed.On the other hand, the resolving power of the system turns out to beincreased by at least 30 percent more than that of the conventionalsystem.

Following the methods known to those skilled in the art, viz. usingemulsions having a grain size ranging from 0.7 to 0.8 microns andsuitable gelatin substitutes capable of increasing the covering power ofthe developed silver, it has been possible to prepare X-ray films havingan approximately 40 percent lower silver coating weight than the X-raymaterial used in the compared conventional system. Although thesensitivity of such X-ray films turns out to be reduced by more than onehalf, the sensitivity increase obtained by means of theimage-intensifier of the present invention still allows exposure timesor dosages lower than those re quired by the abovementioned'conventional system.

The following examples are illustrative of the methods and compositionsof the present invention but should not be construed as limiting thescope of the present invention.

EXAMPLE 1 According to known techniques a less than 1 micron thick tinoxide layer was deposited on a 1.5 mm glass base (see for example R.Gomer, The Review of Scientific Instruments, p. 993 V24 (1953)). Anapproximately 100 micron thick photoconducting layer formed byphotoconductive grade copper doped cadmium selenide grains obtained fromE.S.P.l. (Electronic Space Products, Inc.) of California, U.S.A., andsubsequently doped with 5 ppm. of aluminum dispersed in a nitrocellulosebinder, was coated on the above mentioned conducting layer. An opaquelayer of thickness about microns made from particles of titanium dioxidedispersed in Cyanocel was coated on top of the photoconducting layer. Anapproximately 30 micron thick electroluminescent layer consisting ofcopper doped zinc sulphide powder type EL CB3 of Levy- WestLaboratories, Bush Fair, Harlow, Essex, England, dispersed in Cyanocelwas coated on the opaque layer. An approximately 100 A thicksemi-transparent electrode consisting of chromium was deposited byvacuum evaporation on the electroluminescent layer. A protective layerof thickness approximately 10 microns thick consisting of a polyurethaneenamel made of Desmophen 650 -Desmodur as described in point (7) above,was coated on the semi-transprent electrode. Two conductors connectedthe two electrodes to an alternating current source, which was in thiscase a normal 220 volt 50 Hz power point.

The image amplifying element, thus prepared, was exposed to X-rayradiations generated in a Machlett OEG-50-T type X-ray tube working at avoltage of 40kV and a current of IOmA. The distance between the sourceand the sample was cm. A 7 mm. thick aluminum filter had been placedbetween the X-ray source and the intensifier.

In such conditions, we obtained a photon emission increase l0 timeshigher than the one obtained with a DuPont Par-Speed Screen.

The photographic test was carried out as follows: An XV 5 X-ray film of3M ltalia S.p.A. was exposed as shown in FIG. 1, wherein: A is the X-raysource; B is the aluminum filter; C is the image intensifier accordingto the present invention; and D is the X-ray film.

Layers C and D are in intimate contact but are shown separately forclarity. The sensitivity of the system of the present example can bemeasured with one of the methods known to those skilled in the art, suchas for instance an aluminum step wedge having 2 mm. steps.

The sensitivity, thus measured, has been compared with that obtained byexposing the above defined conventional system to the same X-ray source.The results,

thus obtained, show a sensitivity increase of the system,

by a factor of at least 6.

' EXAMPLE 2 Example 1 was performed again, with the exception that aGevaert Scopic l S Film of Gevaert-Agfa N.V. was used instead of the XV5 Film of 3M ltalia S.p.A. The electroluminescent compound used for thepreparation of the image intensifier was the EL-CB4 type of Levy West.Also in this case, the sensitivity resulted to be 6 times higher thanthe sensitivity obtained with the conventional system described inExample 1.

We claim:

1. An X-ray image intensifying device for radiography comprising thefollowing sequential layers:

A. A supporting film base,

B. A conducting layer which is at least partially transparent to X-rays,

C. An X-ray sensitive photoconductive layer,

D. A light opaque layer,

E. An electroluminescent layer comprising an electroluminescent compounddispersed in a binder,

F. An at least partially light-transparent electrically conductinglayer, and

G. A protective layer whose thickness is such that the resolving powerloss is not greater than 20 percent lower than that of the same elementwithout any protective layer.

2. The X-ray image intensifying device of claim 1 wherein the thicknessof said protective layer is not more than 50 microns.

3. A device for radiography comprising a lightsensitive recordingelement positioned in close contact with the intensifying device ofclaim 1.

4. The image intensifier of claim 1 having a wavelength emission maximumlonger than 500 nm, combined with an X-ray film containing a sensitizerimparting a sensitization maximum substantially in the same emissionrange of the image intensifier.

5. The image intensifier in claim 1 combined with an X-ray filmparticularly sensitive to those wavelengths emitted by the saidintensifier.

6. The X-ray image intensifying device of claim 1 wherein the supportingbase is also a conducting layer at least partially transparent to'X-rays.

7. An X-ray image intensifying device for radiography comprising thefollowing sequential layers:

A. A supporting base, at least percent transparent to X-rays, having athickness no greater than about 300 microns,

B. An electrically conductive layer which is at least 90 percenttransmissive of 20 kV X-rays,

C. An X-ray sensitive photoconductive layer the resistance of whichvaries according to the incident X-ray intensity and which resistancevaries by at least a factor of 10 when irradiated by X-rays produced bya tungsten target X-ray tube operated at 40 kV and 1 mA at a distance of20 cm, the radiation from the said tube being emitted through a 0.040inch thick beryllium window with a 5 mm. focal spot, the said electronconductive layer being not greater than 200 microns in thickness,

D. A light opaque layer having an impedance lower than that of theelectroluminescent layer,

E. An electroluminescent layer having a dielectric constant of at least5 and having an impedance 8. The X-ray image intensifying device ofclaim 2 wherein the thickness of said protective layer is not more than50 microns.

9. The X-ray intensifying devices of claim 8 wherein the light opaquelayer has an impedance lower than that of the electroluminescent layerby at least a factor of 10.

10. The X-ray image intensifying device of claim 7 wherein thesupporting base (A) is also the electrically conductive layer (B).

1. AN X-RAY IMAGE INTENSITYING DEVICE FOR RADIOGRAPHY COMPRISING THEFOLLOWING SEQUENTIAL LAYERS: A. A SUPPORTING FILM BASE, B. A CONDUCTINGLAYER WHICH IS AT LEAST PARTIALLY TRANSPARENT TO X-RAYS, C. AN X-RAYSENSITIVE PHOTOCONDUCTIVE LAYER, D. A LIGHT OPAQUE LAYER, E. ANELECTROLUMINESCENT LAYER COMPRISING A ELECTROLUMONESCENT COMPOUNDDISPERSED IN A BINDER, F. AN AT LEAST PARTIALLY LIGHT-TRANSPARENTELECTRICALLY CONDUCTING LAYER, AND G. A PROTECTIVE LAYER WHOSE THICKNESSIS SUCH THAT THE RESOLVING POWER LOSS IS NOT GREATER THAN 20PERCENTLOWER THAN THAT OF THE SAME ELEMENT WITHOUT ANY PROTECTIVELAYER.
 1. An X-ray image intensifying device for radiography comprisingthe following sequential layers: A. A supporting film base, B. Aconducting layer which is at least partially transparent to X-rays, C.An X-ray sensitive photoconductive layer, D. A light opaque layer, E. Anelectroluminescent layer comprising an electroluminescent compounddispersed in a binder, F. An at least partially light-transparentelectrically conducting layer, and G. A protective layer whose thicknessis such that the resolving power loss is not greater than 20 percentlower than that of the same element wIthout any protective layer.
 2. TheX-ray image intensifying device of claim 1 wherein the thickness of saidprotective layer is not more than 50 microns.
 3. A device forradiography comprising a light-sensitive recording element positioned inclose contact with the intensifying device of claim
 1. 4. The imageintensifier of claim 1 having a wavelength emission maximum longer than500 nm, combined with an X-ray film containing a sensitizer imparting asensitization maximum substantially in the same emission range of theimage intensifier.
 5. The image intensifier in claim 1 combined with anX-ray film particularly sensitive to those wavelengths emitted by thesaid intensifier.
 6. The X-ray image intensifying device of claim 1wherein the supporting base is also a conducting layer at leastpartially transparent to X-rays.
 7. An X-ray image intensifying devicefor radiography comprising the following sequential layers: A. Asupporting base, at least 90 percent transparent to X-rays, having athickness no greater than about 300 microns, B. An electricallyconductive layer which is at least 90 percent transmissive of 20 - 150kV X-rays, C. An X-ray sensitive photoconductive layer the resistance ofwhich varies according to the incident X-ray intensity and whichresistance varies by at least a factor of 10 when irradiated by X-raysproduced by a tungsten target X-ray tube operated at 40 kV and 1 mA at adistance of 20 cm, the radiation from the said tube being emittedthrough a 0.040 inch thick beryllium window with a 5 mm. focal spot, thesaid electron conductive layer being not greater than 200 microns inthickness, D. A light opaque layer having an impedance lower than thatof the electroluminescent layer, E. An electroluminescent layer having adielectric constant of at least 5 and having an impedance lower thanthat of the non-irradiated photoconductive layer, but not less than 10percent of the impedance of said non-irradiated photoconductive layer,F. An electrically conducting layer which is at least 50 percenttransmissive to light at wavelengths between 500 and 550 nm, and G. Aprotective layer whose thickness is such that the resolving power lossis not more than 20 percent lower than that of the same element withoutany protective layer.
 8. The X-ray image intensifying device of claim 2wherein the thickness of said protective layer is not more than 50microns.
 9. The X-ray intensifying devices of claim 8 wherein the lightopaque layer has an impedance lower than that of the electroluminescentlayer by at least a factor of 10.